In orthopedic and orthodontic procedures the need often arises to fix an implant to bone. The most common method for such fixation is carried out by using various types of pins and/or screws commonly designed for a specific orthopedic or orthodontic procedure.
One such procedure, for example, would be to fix joint articulation surface implants to the underlying bone as part of an arthroplasty procedure such as described in US Patent Application Publication No. 2009\0222103. Another example for fixation of a bone plate to a bone using screws is described in U.S. Pat. No. 5,578,034. U.S. Pat. No. 3,466,478 discloses an anchor screw for fixating an orthodontic prosthesis to the jaw bone.
In arthroplasty, it is very important for the implant to remain firmly attached to the underlying bone over time. Fixation of such implants to bone have originally employed cement. Later cementless fixation techniques, mainly by employing screws and pins have been developed.
In many cases, however, fixation devices such as screws, pins or similar keeping implants such as arthroplasty implants in place are embedded in the bone at locations commonly subject to high stress such as compression and shearing forces, bending forces, torque or a combination of all. In arthroplasty, for example, the bone joint articulation surfaces can be such a location. An articulating joint by definition is subjected to high stress such as forces of compression and shearing resulting from the shift of the load-bearing articulating surfaces during movement of the articulating bones such as during walking.
Another such location can be the jaw bone in which an orthodontic insert can be placed in a jaw bone supporting an implant. Here too, the orthodontic insert must withstand significant excessive compressive and bending forces transferred thereto from the implant during mastication.
The stress to which the implants are subjected many times impacts the bone screw or pin by bringing about failure of the fixation device. Such failure commonly exhibits itself in the form of loosening, device fatigue and axial pull-out of the device, i.e., axial forces acting, for example on a screw and translated into rotational forces that cause the device to unscrew and loosen bringing about irreversible loss of the bone-implant interface. Since the thread created in the bone cortex by the commonly used screws is relatively shallow, in some cases the bony thread itself may strip and the fixating device can lose its holding power or grip.
Several attempts have been made to overcome the above described failures. U.S. Pat. No. 6,575,975 discloses one example of a fixation device comprising a bushing having a locking screw that is threaded through the head of the bushing expanding the radial walls of the head and locking the bushing in place.
U.S. Pat. No. 5,716,358 discloses an orthopedic bone screw having directional asymmetry and asymmetrical surface roughness provided by a plurality of oriented microstructures on the surface.
Another contributing factor to fixation devices premature loosening and failure is the shoulder of the fixation device such as a bone plate lag screw. Lag screws are commonly designed to slip along the shoulder of screw hole in a bone plate hence the shoulders of the lag screw are rounded. Rounded shoulders allow for pendulous movement (“rocking”) of the bone over time in response to the various forces of stress discussed above. This pendulous movement eventually leads to loosening of the screw or similar fixation device leading to failure.
Failure may not necessarily be mechanical in nature. Bringing two dissimilar conducting materials, such as metals, in contact with each other leads to an electrochemical potential difference between them and a development of galvanic corrosion. Aggressive corrosion resulting from an electrical circuit established between the two different metals one of which becomes an anode while the other—a cathode. Common sense would dictate not using multiple metals in an orthopedic implant.
In most cases, the fixation of an implant to bone is expected to be permanent or at least long-term. Unfortunately, this goal often remains unfulfilled for the reasons disclosed above.
Since the types of stress to which the fixation device can be subjected might vary in nature from location to location, oftentimes the proposed fixation devices are designed in an attempt to overcome fixation device failures in a specific location. Such designs require the orthopedic surgeon, dentist or medical institution to stock a variety of types of fixation devices which can become quite expensive. A single type universal fixation device that can be scaled up or down to various sizes suitable for various orthopedic as well as orthodontic procedures could provide a solution to overcome such deficiencies and bring about a reduction in manufacturing costs as well as expenses for the surgeon, dentist or medical institution.